1. Field of the Invention
The present invention relates to a foldback limiting circuit and a power regulator using the same, more particularly to, a voltage regulator having an active foldback current limiting circuit.
2. Description of the Prior Arts
Refer to FIG. 1 and FIG. 2, which illustrate the conventional approaches.
Generally speaking, during the application of DC voltage regulator (power regulator), there are always some protection circuits, which can be categorized by an over voltage protection, an over temperature protection, and a short circuit protection. As the short circuit protection is concerned, a foldback current limiting circuit can realize the same. The mechanism for the foldback current limitation, in the most of occasions, is to take advantage of piece-wisely changing the size of detecting current so as to achieve a relatively smaller limiting current.
The disclosures illustrated in FIG. 1 and FIG. 2, are the prior arts for the foldback current limiting circuit. In FIG. 1, a transistor M102 is used to sense a current flowing through a power transistor M101, at the time for the case of over current, a voltage drop across RS101 is adequate to turn on a transistor M105 so as to generate a charging current to clamp the gate voltage VEO1 and the initial purpose of current limiting can be achieved.
A transistor M106 and a resistor RS102 illustrated in FIG. 1 are a part of the foldback current limiting circuit, which serve the purpose of short-circuited current protection. While a short circuit situation happening at the output voltage side, said M106 will be turned off and the current flowing through RS101 will be increasing, therefore, the charging current for said M105 is also increasing accordingly such that the gate voltage of said M101 will be clamped at an even higher voltage reference so as to limit the short circuit at a lower state.
As suggested by FIG. 2, while a transistor M202 detects the over current situation for a power transistor M201, a voltage drop across a resistor R203 is adequate to turn on a transistor M220 and further take advantage of a resistor R205 to convert the current flowing through a transistor M222 into a voltage and further turn on a transistor M203 to generate a charging current to clamp the gate voltage of M201 so as to achieve the initial purpose of current limiting as FIG. 1 suggests.
However, said RS101, R203, and R205 in FIG. 1 and FIG. 2, as well as said M220 in FIG. 2 are vulnerable to process and temperature variation and influencing directly the accuracy of short circuit limiting current. Additionally, since in both of the prior arts the resistors are inevitable, if willing to limit the current at a lower value, the corresponding resistance must be increased. In the disclosure of FIG. 1, the turning-on impedance introduced by the transistor M106 needs to be further considered, that is to say, if said impedance is exceedingly large, then the normal operation of the voltage regulator cannot function properly. To sum up, to enhance the accuracy for diversified process and temperature variation, and the usage of the area efficiency for die area, are both the topics of the present invention.
Accordingly, in view of the above drawbacks, it is an imperative that a foldback current limiting circuit, especially an active foldback current limiting circuit for a power regulator is designed so as to solve the drawbacks as the foregoing.